The present invention relates to systems and methods which provide brightness control processing for images received from an image sensor. More particularly, the present invention relates to imaging systems for optical vehicle safety systems or driver assistance systems.
Electronic imaging systems have long been used to provide images in various applications. One of the primary problems that imaging systems must overcome relates to variable scene brightness within the imager field-of-view. Because the brightness level in a scene to be imaged can often change dramatically, the image sensor must adjust the exposure/gain levels to compensate for the changing brightness level. Slow but very significant variations will obviously occur between day and night operation but more rapid variations may also occur. For example, one application of image sensing systems is in the field of vehicle safety or navigation systems. More particularly, image sensing systems can be used to monitor a vehicle""s progress in a desired lane of traffic, and to issue a warning to the driver of the vehicle if it is unintentionally exiting the designated lane. In such applications, extreme and rapid brightness changes in the scene may be experienced due to tunnel exits, irregular shadows on the road, etc., along with the slower variations in brightness due to time of day and weather conditions.
Typically, one of the problems encountered due to brightness variation in the scene to be imaged is that the image sensor only has a certain dynamic range. Without control over image sensor exposure/gain, extreme brightness will thus easily saturate the image sensor while an extremely dark scene will fall below the minimum detectable light level. Therefore, one goal of the system must be to continually modify the exposure/gain settings of the image sensor in accordance with the changing brightness level in the scene.
Typically, the maximum dynamic range of an imaging system is obtained by varying the exposure/gain of the image sensor in the system. Such image sensor based systems are increasingly single chip VLSI devices. Therefore, in one approach to exposure/gain control, the imager chip manufacturer permanently encodes an exposure/gain control algorithm on the chip. In such an approach, the on chip algorithm first counts all pixels in the image which have an intensity greater than a set threshold level of brightness. Based on this count, the on chip algorithm decides whether to increase or decrease the amount of exposure/gain for the sensor. The on chip algorithm may alter the exposure/gain value by either large or small increments, depending on the relative difference between the bright pixel count and the threshold values.
One example of such a prior art imaging chip with on chip brightness compensation is the VV5430 Monolithic Sensor which is manufactured by VLSI Vision, Ltd. in the United Kingdom. This chip counts all pixels in the image which have an intensity greater than a certain Very White Pixel threshold. This prior art approach then changes the exposure and gain settings based on a comparison of this count to fixed brightness thresholds.
In this prior art approach, the exposure/gain is optimized for an entire field-of-view. However, some applications can require optimization only in small region(s) of interest in a scene. If the overall scene brightness is much different from the brightness in the region(s) of interest, the above-described dynamic exposure/gain control method can actually degrade the brightness and contrast in the region(s) of interest in an image.
Another aspect of this prior art approach is that it employs one fixed set of brightness thresholds. These thresholds are chosen to provide images acceptable for most applications in most lighting conditions, but may provide poor quality images in extreme lighting conditions, such as glare, shadows, or overall dark or bright extremes of light. Some applications require images with optimal brightness and contrast even in these extreme conditions, in which case, the above-described method is not adequate.
Also, this prior art approach is designed to adjust for changes in brightness somewhat gradually. In an environment where brightness may be rapidly changing, such adjustments allow for many image frames which are somewhat overexposed or underexposed. For example, glare or shadows moving through a scene may suddenly change the brightness of a portion of the scene very significantly. If the image system is being used in a computer system application where such glare or shadow effect is a factor, the image frames with less than optimal brightness and contrast may need to be either disregarded or processed further using algorithms for correcting severely degraded image quality which can significantly reduce system performance.
Therefore, a need presently exists for a method which provides dynamic brightness control of an image sensor and addresses the problems noted above.
The present invention provides an imaging system and method for brightness control of an image sensor which optimizes the dynamic range of the sensor for one or more regions of interest within the imager field-of-view.
The present invention further provides an imaging system and method for brightness control of an image sensor which optimizes exposure/gain of the sensor for a wide range of lighting conditions including very dark or very bright.
The present invention further provides an imaging system and method for brightness control of an image sensor which adapts quickly to changing light conditions.
In a preferred embodiment, the present invention provides a method of brightness control of an image sensor based on image data received from the image sensor. One or more object(s) of interest within the image are identified and the brightness level of one or more region(s) in the image is detected, said region(s) comprising the background surrounding said object(s) of interest. An adjusted brightness level of the image is determined such that the brightness of the object(s) of interest will be optimized in relation to the background region(s) surrounding said object(s) of interest. The image sensor settings are controlled to alter the brightness level in the image region(s) to the adjusted brightness level to enhance imaging of the object(s) of interest. In another aspect, the present invention includes an imaging system having an image sensor for providing image data of a scene. An image control block includes means for identifying one or more object(s) of interest in said image and means for detecting the brightness level of one or more region(s) in the image, said region(s) comprising the background surrounding said object(s) of interest. The image control block also includes means for determining an adjusted brightness level of the image, such that the brightness of said object(s) of interest is optimized in relation to the background region(s) surrounding said object(s) of interest. The image control block also includes control means coupled to the image sensor, for controlling the image sensor to alter the brightness level of the image to the adjusted brightness level to enhance imaging of the object(s) of interest in the scene.
A more complete understanding of the method and apparatus for an image sensor brightness control will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiments. Reference will be made to the appended sheets of drawings which will first be described briefly.